For current integrated circuit (IC) technology, the speed limiting factor is no longer the transistor gate delay, but the RC delays associated with the interconnects. For this reason, a great deal of work has been done on developing new low dielectric constant materials to reduce interconnect capacitance. Some of these dielectrics include fluorinated silicon dioxide, polymers, and xerogels. However, these material currently pose numerous reliability, manufacturability, and integration issues. Some of these include 1) mechanical strength; 2) dimensional stability; 3) thermal stability; 4) ease of pattern and etch; 5) thermal conductivity; and 6) chemical-mechanical polish (CMP) compatibility. Most low dielectric constant materials under investigation are, as currently developed, inferior to the currently used intermetal dielectric material, silicon dioxide, in most if not all of the above properties.
Also, as IC's continue to scale, the intralevel line-to-line capacitance increasingly dominates over the interlevel capacitance. Thus, it becomes increasingly important that the low dielectric constant material be used between adjacent metal lines and less so between metal levels. Moreover, since tighter metal spacings have increased capacitance, the need for low dielectric constant material there is greater.